Multi-channel communication systems are often susceptible to interference between the various channels, also referred to as crosstalk or inter-channel crosstalk. For example, digital subscriber line (DSL) broadband access systems typically employ discrete multi-tone (DMT) modulation over twisted-pair copper wires. One of the major impairments in such systems is crosstalk between multiple subscriber lines within the same binder or across binders. Thus, signals transmitted over one subscriber line may be coupled into other subscriber lines, leading to interference that can degrade the throughput performance of the system. More generally, a given “victim” channel may experience crosstalk from multiple “disturber” channels, again leading to undesirable interference.
Different techniques have been developed to mitigate, suppress or otherwise control crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectrum management techniques to multi-channel signal coordination.
By way of example, certain of the above-noted techniques allow active cancellation of inter-channel crosstalk through the use of a precoder. In DSL systems, the use of a precoder is contemplated to achieve crosstalk cancellation for downstream communications between a central office (CO) or another type of access node (AN) and customer premises equipment (CPE) units or other types of network terminals (NTs). It is also possible to implement crosstalk control for upstream communications from the NTs to the AN, using so-called post-compensation techniques implemented by a postcoder. Such pre-compensation and post-compensation techniques are also referred to as “vectoring,” and include G.vector technology, which was recently standardized in ITU-T Recommendation G.993.5.
One known approach to estimating crosstalk coefficients for downstream or upstream crosstalk cancellation in a DSL system involves transmitting distinct pilot signals over respective subscriber lines between an AN and respective NTs of the system. Error feedback from the NTs based on the transmitted pilot signals is then used to estimate crosstalk. Other known approaches involve perturbation of precoder coefficients and feedback of signal-to-noise ratio (SNR) or other interference information.
Crosstalk estimates are commonly utilized in situations where one or more inactive lines are being activated in a DSL system. The lines that are being activated are referred to as “activating lines” or “joining lines.” For example, it may become necessary to activate one or more inactive lines in a synchronization group that already includes multiple active lines, where synchronization in this context refers to alignment in time of the DMT symbols for the different lines. Such activating of an additional line may require that the crosstalk compensation be adjusted accordingly in order to optimize system performance. Exemplary techniques for controlling crosstalk associated with a joining line are disclosed in European Patent Application Publication No. EP 1936825A1, entitled “A Transient Crosstalk Controlling Device,” which is incorporated by reference herein. Crosstalk estimates are also used in other situations, e.g., as a means to track changes in crosstalk over time.
In conventional DSL systems, it can be difficult to generate sufficiently accurate crosstalk estimates in the presence of impulse noise. Impulse noise is known to have an adverse impact on data reception, and standardized channel codes, such as Reed-Solomon codes, are typically utilized to alleviate this adverse impact. Nonetheless, impulse noise remains a significant problem in pilot signal aided estimation of crosstalk. For example, even a single impulse occurring during crosstalk estimation can degrade the estimates so severely that there is a significant SNR loss caused when the estimates are used for vectoring. Crosstalk estimates based on error feedback techniques are particularly vulnerable to such impulse noise. Standard error feedback techniques transmit the above-noted distinct pilot signals using sync symbols which occur 16 times per second. If even a single sync symbol is corrupted by impulse noise, the resulting crosstalk estimates may be extremely poor.